Gas turbines have been in use for electric power production for decades. Over that time, the industry has changed significantly and this has driven a change in the desired attributes and capabilities for gas turbines used for power production. In the 1980's and early 1990's, gas turbines were primarily used in base load operation. Since that time operational requirements have changed and gas turbines are used to load follow, changing operating load frequently over time. This requirement continues to expand and with the recent introduction of a larger portfolio of wind and other sources where ramp can not be completely controlled or predicted, it is desirable to have gas turbines which are able to ramp up and down quickly. To address the demands of base load, fluctuating load and peaking load, various sizes of gas turbines are employed to be able to address the desired amount of power required for each demand.
Industrial gas turbines have historically been used to produce as much power as they could, resulting in engine designs that operate at maximum efficiency at the maximum load. This approach to gas turbine design is in large part based on the efficiency characteristics of the gas turbine. For a given gas turbine design, the efficiency curve has a characteristic shape. From an aerodynamic perspective, from the lowest operating power point to the maximum operating point, the efficiency curve slopes slowly up to a maximum efficiency. However, once the maximum efficiency is reached, it drops sharply with further increases in power. This drop is due to aerodynamic losses that occur in the turbine when there is flow beyond the optimum design for a row of airfoils. Due to this phenomenon, it has been industry practice to design and/or control the engines such that they reach their maximum power at a point very near to, but not past their maximum efficiency.
In the past, gas turbines operated as base load units, or for peaking power supply for short periods. Base load units ran at maximum power for extended periods. Peakers were expected to cycle frequently in order to supply additional power when demand increased. A regional power grid in the U.S. might be supplied by a combination of nuclear, coal, wind, and gas-fired generation, with gas ramping on and off or up and down for peaks in demand during the day, including providing the additional power requirements of the power grid as the output of renewable sources, e.g., solar and wind, fluctuate.
As the need for more operational flexibility was introduced, engine designs to flatten the efficiency curve emerged and some industrial designs started to introduce some of the features from aircraft engines into industrial engines. Among these features was the introduction of more than one row of compressor variable vanes. In many industrial gas turbine designs, only one row of vanes is used to adjust the mass flow of air entering the compressor. When only one row is used, large mass flow reductions result in large reductions in compressor efficiency. When more than one row is used, the compressor mass flow can be changed significantly without significantly changing the compressor efficiency. Though an improvement, this lower loss in efficiency in the compressor with mass flow does not change the way the turbine reacts to the mass flow, and gas turbines are generally currently designed and/or controlled such that the maximum power and maximum efficiency are very close.